This invention relates to improvements in power assist steering systems having primary and auxiliary fluid pressure supply pumps.
An auxiliary power steering system according to U.S. Pat. No. 3,747,725, is supplied with fluid through a pump transfer valve having a pressure face directly exposed to pressurized fluid from a primary or engine driven pump. An opposing spring-loaded pressure face of the valve assembly also has pressurized fluid applied thereto through an orifice assembly from the primary pump. The transfer valve has two stable positions and an unstable intermediate position. As long as the engine driven pump operates, it will supply a power steering unit with pressurized fluid while the emergency or auxiliary steering pump, driven by the vehicle is unloaded. The transfer valve in this operational mode is in one of its stable positions. In the event there is a drop in the flow rate of fluid delivered through the orifice assembly below a certain value, the transfer valve is displaced to the other stable position through the unstable intermediate position in which the emergency steering pump takes over so as to supply pressurized fluid to the power steering unit. In the intermediate position of the transfer valve, the engine driven pump is momentarily unloaded so that the separation of the two fluid circuits respectively associated with the two pumps is disturbed. Also, precisely timed pump change-over by the transfer valve for automatic fluid supply purposes is made difficult during partial failure of the engine driven pump because a residual pressure remains in the pump outlet line, the effect of which cannot be cancelled by the spring bias applied to the opposing pressure face of the transfer valve.
Other known prior art steering arrangements likewise operate with such a pressure dependent transfer valve assembly which may not change-over to reserve pump operation if a residual pressure output of the engine driven pump, of for example 10 BAR persists, since the bias of the spring acting on an opposing side of the transfer valve assembly is not sufficient to cause valve shift.